The work does not arrive evenly. It arrives all at once.
Two clients peak in the same week. A pitch demands more hands than the roster holds. Someone key leaves at the wrong moment. The demand is there, and the team to meet it is not, and hiring for it takes months you do not have.
That is the squeeze every agency knows. Turn work away, or stretch the team until the quality slips.
There is a third option. Have people you can call on. Senior operators and teams you can add fast, for the peak, the pitch, or the gap, without carrying them on payroll through the quiet stretches.
The agencies that never get caught short are not the ones who over-hire. They are the ones with somewhere to reach when the work comes in faster than they can staff it.
AP is that reach. The talent an agency can pull in when the work outpaces the roster, and let go of when it settles.